1. Lecture 4: Physical Layer & Link Layer Basics Based on slides from previous 441 lectures 15-441: Computer Networking Copyright © CMU, 2007-2011.

2. F'11Lecture 4: Physical Layer2 Last Time Application Layer Example Protocols ftp http Performance Application Presentation Session Transport Network Datalink Physical

3. F'11Lecture 4: Physical Layer3 Today (and beyond) Physical layer. Datalink layer introduction, framing, error coding, switched networks. Broadcast-networks, home networking. Application Presentation Session Transport Network Datalink Physical

4. F'11Lecture 4: Physical Layer4 Transferring Information Information transfer is a physical process In this class, we generally care about Electrical signals (on a wire) Optical signals (in a fiber) More broadly, EM waves Information carriers can also be ?

5. F'11Lecture 4: Physical Layer5 Transferring Information Information transfer is a physical process In this class, we generally care about Electrical signals (on a wire) Optical signals (in a fiber) More broadly, EM waves Information carriers can also be Sound waves Quantum states Proteins Ink & paper, etc.

6. F'11Lecture 4: Physical Layer6 From Signals to Packets Analog Signal “Digital” Signal Bit Stream 0 0 1 0 1 1 1 0 0 0 1 Packets 0100010101011100101010101011101110000001111010101110101010101101011010111001 Header/Body ReceiverSender Packet Transmission Application Presentation Session Transport Network Datalink Physical

7. F'11Lecture 4: Physical Layer7 From Signals to Packets Analog Signal “Digital” Signal Bit Stream 0 0 1 0 1 1 1 0 0 0 1 Packets 0100010101011100101010101011101110000001111010101110101010101101011010111001 Header/Body ReceiverSender Packet Transmission

8. F'11Lecture 4: Physical Layer8 Today’s Lecture Modulation. Bandwidth limitations. Frequency spectrum and its use. Multiplexing. Media: Copper, Fiber, Optical, Wireless. (Next Week Coding. Framing.)

9. F'11Lecture 4: Physical Layer9 Why Do We Care? Get the big picture. Physical layer places constraints on what the network infrastructure can deliver Reality check Impact on system performance Impact on the higher protocol layers Some examples: Fiber or copper? Do we need wires? Error characteristic and failure modes Effects of distance

10. F'11Lecture 4: Physical Layer10 Baseband vs Carrier Modulation Baseband modulation: send the “bare” signal. Carrier modulation: use the signal to modulate a higher frequency signal (carrier). Can be viewed as the product of the two signals Corresponds to a shift in the frequency domain

11. F'11Lecture 4: Physical Layer11 Modulation Changing a signal to convey information From Music: Volume Pitch Timing

12. F'11Lecture 4: Physical Layer12 Modulation Changing a signal to convey information Ways to modulate a sinusoidal wave Volume:Amplitude Modulation (AM) Pitch:Frequency Modulation (FM) Timing:Phase Modulation (PM) In our case, modulate signal to encode a 0 or a 1. (multi-valued signals sometimes)

13. F'11Lecture 4: Physical Layer13 Amplitude Modulation AM: change the strength of the signal. Example: High voltage for a 1, low voltage for a 0 0 0 1 1 0 0 1 1 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 1 1 0 1 0 1 0 1

14. F'11Lecture 4: Physical Layer14 Frequency Modulation FM: change the frequency 0 1 1 0 1 1 0 0 0 1

15. F'11Lecture 4: Physical Layer15 Phase Modulation PM: Change the phase of the signal 1 0 1 0

16. F'11Lecture 4: Physical Layer16 Amplitude Carrier Modulation Amplitude Signal Carrier Frequency Amplitude Modulated Carrier

17. F'11Lecture 4: Physical Layer17 Why/Why Not Baseband? Baseband Simpler hardware Modulated Carrier Control the range of frequencies used Wire and electronics at 100Hz can behave very differently from wire and electronics at 10MHz Amplitude Baseband Modulated Carrier freq.  few percent  well-behaved and easy to process Highest frequencies many times higher than low frequencies  may cause problems

18. F'11Lecture 4: Physical Layer18 Why Different Modulation Methods?

19. F'11Lecture 4: Physical Layer19 Why Different Modulation Methods? Transmitter/Receiver complexity Power requirements Bandwidth Medium (air, copper, fiber, …) Noise immunity Range Multiplexing

20. F'11Lecture 4: Physical Layer20 What Do We Care About? Cost How much bandwidth can I get out of a specific wire (transmission medium)? What limits the physical size of the network? How can multiple hosts communicate over the same wire at the same time? How can I manage bandwidth on a transmission medium? How do the properties of copper, fiber, and wireless compare?

21. F'11Lecture 4: Physical Layer21 Bandwidth Bandwidth is width of the frequency range in which the Fourier transform of the signal is non- zero. (At what frequencies is there energy?) Sometimes referred to as the channel width Or, where it is above some threshold value (Usually, the half power threshold, e.g., -3dB) dB Short for decibel Defined as 10 * log 10 (P 1 /P 2 ) When used for signal to noise: 10 * log 10 (P S /P N )

22. F'11Lecture 4: Physical Layer22 Signal = Sum of Waves = + 1.3 X + 0.56 X + 1.15 X

23. F'11Lecture 4: Physical Layer23 The Frequency Domain A (periodic) signal can be viewed as a sum of sine waves of different strengths. Corresponds to energy at a certain frequency Every signal has an equivalent representation in the frequency domain. What frequencies are present and what is their strength (energy) E.g., radio and TV signals.

24. F'11Lecture 4: Physical Layer24 A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H. Assumes binary amplitude encoding: 1  1.0, 0  -1.0 The Nyquist Limit 0 0 1 0 0 0 1 1 1 0 0 0 1 0 1 0 0 1 1 0

25. F'11Lecture 4: Physical Layer25 0 0 1 0 0 0 1 1 1 0 0 0 1 0 1 0 0 1 1 0 A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H. Assumes binary amplitude encoding E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second? The Nyquist Limit Hmm, I once bought a modem that did 54K????

26. F'11Lecture 4: Physical Layer26 How to Get Past the Nyquist Limit Instead of 0/1, use lots of different values. (Remember, the channel is noiseless.) Can we really send an infinite amount of info/sec? 0 0 1 0 0 0 1 1 1 0 ? 0 1 0 1 0 0 1 1 0

27. F'11Lecture 4: Physical Layer27 Past the Nyquist Limit Every transmission medium supports transmission in a certain fixed frequency range. The channel bandwidth is determined by the transmission medium and the quality of the transmitter and receivers. More aggressive encoding can increase the channel bandwidth... to a point...

28. F'11Lecture 4: Physical Layer28 Capacity of a Noisy Channel Can’t add infinite symbols you have to be able to tell them apart. This is where noise comes in.

29. F'11Lecture 4: Physical Layer29 Capacity of a Noisy Channel Can’t add infinite symbols you have to be able to tell them apart. This is where noise comes in. Shannon’s theorem: C = B x log 2 (1 + S/N) C: maximum capacity (bps) B: channel bandwidth (Hz) S/N: signal to noise (power) ratio of the channel Often expressed in decibels (db) ::= 10 log(S/N).

30. F'11Lecture 4: Physical Layer30 Capacity of a Noisy Channel Can’t add infinite symbols you have to be able to tell them apart. This is where noise comes in. Shannon’s theorem: C = B x log 2 (1 + S/N) C: maximum capacity (bps) B: channel bandwidth (Hz) S/N: signal to noise ratio of the channel Often expressed in decibels (db) ::= 10 log(S/N) Example: Local loop bandwidth: 3200 Hz Typical S/N: 1000 (30db) What is the upper limit on capacity? 3200 x log 2 (1 + 1000) = 31.895 kbits/s

31. F'11Lecture 4: Physical Layer31 Transmission Channel Considerations Every medium supports transmission in a certain frequency range. Outside this range, effects such as attenuation degrade the signal too much Transmission and receive hardware will try to maximize the useful bandwidth in this frequency band. Tradeoffs between cost, distance, bit rate As technology improves, these parameters change, even for the same wire. Frequency GoodBad Signal

32. F'11Lecture 4: Physical Layer32 Limits to Speed and Distance Noise: “random” energy is added to the signal. Attenuation: some of the energy in the signal leaks away. Dispersion: attenuation and propagation speed are frequency dependent. (Changes signal shape) Effects limit the data rate that a channel can sustain. But affects different technologies in different ways Effects become worse with distance. Tradeoff between data rate and distance

33. F'11Lecture 4: Physical Layer33 Today’s Lecture Modulation. Bandwidth limitations. Frequency spectrum and its use. Multiplexing. Media: Copper, Fiber, Optical, Wireless. (Next Week: Coding. Framing.)

34. F'11Lecture 4: Physical Layer34 Frequency spectrum and its use.

35. F'11Lecture 4: Physical Layer35 Today’s Lecture Modulation. Bandwidth limitations. Frequency spectrum and its use. Multiplexing. Media: Copper, Fiber, Optical, Wireless. (Next Week: Coding. Framing.)

36. F'11Lecture 4: Physical Layer36 Supporting Multiple Channels Multiple channels can coexist if they transmit at a different frequency, or at a different time, or in a different part of the space. Three dimensional space: frequency, space, time Space can be limited using wires or using transmit power of wireless transmitters. Frequency multiplexing means that different users use a different part of the spectrum. Similar to radio: 95.5 versus 102.5 station Controlling time (for us) is a datalink protocol issue. Media Access Control (MAC): who gets to send when?

37. F'11Lecture 4: Physical Layer37 Time Division Multiplexing Different users use the wire at different points in time. Requires spectrum proportional to aggregate bandwidth. Frequency 1 user 2 users

38. F'11Lecture 4: Physical Layer38 FDM: Multiple Channels Amplitude Different Carrier Frequencies Determines Bandwidth of Channel Determines Bandwidth of Link

39. F'11Lecture 4: Physical Layer39 Frequency versus Time-division Multiplexing With FDM different users use different parts of the frequency spectrum. I.e. each user can send all the time at reduced rate Example: roommates With TDM different users send at different times. I.e. each user can send at full speed some of the time Example: time-share condo The two solutions can be combined. Frequency Time Frequency Bands Slot Frame

40. F'11Lecture 4: Physical Layer40 Today’s Lecture Modulation. Bandwidth limitations. Frequency spectrum and its use. Multiplexing. Media: Copper, Fiber, Optical, Wireless. Coding. Framing.

41. F'11Lecture 4: Physical Layer41 Copper Wire Unshielded twisted pair (UTP) Two copper wires twisted - avoid antenna effect Grouped into cables: multiple pairs with common sheath Category 3 (voice grade) versus category 5 100 Mbit/s up to 100 m, 1 Mbit/s up to a few km Cost: ~ 10cents/foot Coax cables. One connector is placed inside the other connector Holds the signal in place and keeps out noise Gigabit up to a km Signaling processing research pushes the capabilities of a specific technology. E.g. modems, use of cat 5 Images: networkcable-tester.com, tootoo.com

42. F'11Lecture 4: Physical Layer42 UTP Why twist wires?

43. F'11Lecture 4: Physical Layer43 UTP Why twist wires? Provide noise immunity Combine with Differential Signaling

44. F'11Lecture 4: Physical Layer44 Light Transmission in Fiber 1000 wavelength (nm) loss (dB/km) 1500 nm (~200 Thz) 0.0 0.5 1.0 tens of THz 1.3  1.55  LEDsLasers

45. F'11Lecture 4: Physical Layer45 Ray Propagation lower index of refraction core cladding (note: minimum bend radius of a few cm)

46. F'11Lecture 4: Physical Layer46 Fiber Types Multimode fiber. 62.5 or 50 micron core carries multiple “modes” used at 1.3 microns, usually LED source subject to mode dispersion: different propagation modes travel at different speeds typical limit: 1 Gbps at 100m Single mode 8 micron core carries a single mode used at 1.3 or 1.55 microns, usually laser diode source typical limit: 10 Gbps at 60 km or more still subject to chromatic dispersion

47. F'11Lecture 4: Physical Layer47 Fiber Types Multimode Single mode

48. F'11Lecture 4: Physical Layer48 Gigabit Ethernet: Physical Layer Comparison MediumTransmit/DistanceComment receive Copper1000BASE-CX25 m machine room use Twisted pair1000BASE-T 100 m four twisted pairs, IEEE 802.3ab MM fiber 62 mm1000BASE-SX260 m 1000BASE-LX500 m MM fiber 50 mm1000BASE-SX525 m 1000BASE-LX550 m SM fiber1000BASE-LX5000 m Twisted pair100BASE-T100 m 2p of UTP5/2-4p of UTP3 MM fiber100BASE-SX2000m

49. F'11Lecture 4: Physical Layer49 How to increase distance? Even with single mode, there is a distance limit. I.e.: How do you get it across the ocean?

50. F'11Lecture 4: Physical Layer50 Even with single mode, there is a distance limit. I.e.: How do you get it across the ocean? How to increase distance? source pump laser

51. F'11Lecture 4: Physical Layer51 Regeneration and Amplification At end of span, either regenerate electronically or amplify. Electronic repeaters are potentially slow, but can eliminate noise. Amplification over long distances made practical by erbium doped fiber amplifiers offering up to 40 dB gain, linear response over a broad spectrum. Ex: 40 Gbps at 500 km. source pump laser

52. F'11Lecture 4: Physical Layer52 Wavelength Division Multiplexing Send multiple wavelengths through the same fiber. Multiplex and demultiplex the optical signal on the fiber Each wavelength represents an optical carrier that can carry a separate signal. E.g., 16 colors of 2.4 Gbit/second Like radio, but optical and much faster Optical Splitter Frequency

53. F'11Lecture 4: Physical Layer53 Wireless Technologies Great technology: no wires to install, convenient mobility, … High attenuation limits distances. Wave propagates out as a sphere Signal strength attenuates quickly  1/d 3 High noise due to interference from other transmitters. Use MAC and other rules to limit interference Aggressive encoding techniques to make signal less sensitive to noise Other effects: multipath fading, security,.. Ether has limited bandwidth. Try to maximize its use Government oversight to control use Huh? 2 in free space, typically 2 to 6

54. F'11Lecture 4: Physical Layer54 Things to Remember Bandwidth and distance of networks is limited by physical properties of media. Attenuation, noise, dispersion, … Network properties are determined by transmission medium and transmit/receive hardware. Nyquist gives a rough idea of idealized throughput Can do much better with better encoding Low b/w channels: Sophisticated encoding, multiple bits per wavelength. High b/w channels: Simpler encoding (FM, PCM, etc.), many wavelengths per bit. Shannon: C = B x log 2 (1 + S/N) Multiple users can be supported using space, time, or frequency division multiplexing. Properties of different transmission media: copper, optical, wireless.